Four-Winged Propeller-Shaped Indole-Modified and Indole-Substituted Tetraphenylethylenes: Greenish-Blue Emitters with Aggregation-Induced Emission Features for Conventional Organic Light-Emitting Diodes

Aggregation-induced emission (AIE) is an extraordinary photochemical phenomenon described by Tang’s group in 2001, where the aggregation of some organic molecules enhances their light emission by limiting intramolecular activity in the aggregate state. This phenomenon offers new opportunities for researchers due to its potential applications in optoelectronics, energy, and biophysics. Tetraphenylethylenes (TPEs) are reliable AIE luminogens with a wide range of successful applications in material chemistry. To expand the library of AIE-active TPEs, both a series of TPE analogues, in which the phenyl rotor has been replaced by the indole ring, and indole-substituted TPE derivatives were designed and synthesized through vinyl–aryl and aryl–aryl bond formations using the Suzuki coupling reaction. Efficient synthetic routes that delivered indole-modified and indole-substituted TPEs have been developed, and almost all heterocyclic TPE analogues have demonstrated AIE behavior. Furthermore, to test whether the indole ring can be diversified, two title compounds were converted to a series of bis(indolyl)methane (BIM), and these BIM–TPE materials showed typical AIE properties. Interestingly, two compounds indicated a solvent vapor fuming reversible switch between bright blue emission and greenish-yellow emission. Upon fuming with dichloromethane, their fluorescence spectra showed 8 and 32 nm red-shift and could return to the original state after fuming with hexane. Furthermore, we have explored the effects of replacing the phenyl ring in TPE with indole together with the substitution of TPE with indole ring(s) on the performance of organic light-emitting diode (OLED) device applications. In addition, density functional theory calculations; the optical, electrochemical, light emission, electroluminescence characteristics; and admittance spectroscopic analysis of OLED devices of four representative TPEs have been investigated in detail. As a result, the indole–TPEs are potential blue emitters with AIE features for conventional OLEDs, which is a significant color in displays and lighting.

General Information. 1 H NMR and 13 C NMR experiments were performed on either 400 MHz Varian and 400 MHz Bruker Avance II instruments using CDCl3, DMSO-d6, MeOD, and Acetone-d6 as the solvent with tetramethylsilane (TMS) as an internal standard at room temperature, and the coupling constants J are given in hertz. The multiplicity is designated as s = singlet, bs = broad singlet, d = doublet, t = triplet, q = quartet, dd = doublet of doublets, m = multiplet. High-resolution mass spectrometry (HRMS) of all compounds was recorded on a QTOF (Quadrupole time-of-flight) spectrometry device. Column chromatography was performed using silica gel pore size 60 Å, 70-230 mesh (sigma).

General procedure for Suzuki coupling = GP1
To a stirred solution of bromo derivative (1 equiv.) and boronic acid or ester derivative (1.1 equiv.) in toluene (40 mL) and EtOH/H2O (4:4 mL) was added Na2CO3 (10 equiv.) and then the system was purged with nitrogen several times. Then Pd(PPh3)4 (0.02 equiv.) was added and the reaction mixture was stirred at 85 °C for 24 h. After cooling to room temperature, the reaction mixture was mixed with water and extracted with dichloromethane (3 × 40 mL) and the organic layers were combined and dried over Na2SO4. After the solvent was removed under reduced pressure, the residue was purified by column chromatography (EtOAc/hexane) to give the desired compound.

General procedure for alkylation = GP2
To a solution of N-H or substituted indoles (1 equiv.) in DMSO (5 mL) sodium hydroxide (1.2 equiv.) was added and stirred for 20-30 minutes at room temperature. Then the flask was placed in an ice bath and alkyl halide (1.2 equiv.) was added slowly for 10 minutes. Removed the ice bath and stirred the reaction mixture for 5 h at room temperature. After completion, the reaction mixture was poured into ice water. Extracted the organic product with ethyl acetate (2 × 30 mL) and dried over anhydrous Na2SO4. Removed the solvent under vacuum and purified the product with column chromatography (EtOAc/hexane) to give the desired compound.

General procedure for BIM-TPE = GP3
To a stirred solution of indole-modified (9) or indole-substituted (31) TPEs (2 equiv.) in CH2Cl2 (15 mL) aldehyde (1 equiv.) and a catalytic amount of Zn(CF3COO)2·xH2O (0.01 equiv.) was added and the mixture was stirred at room temperature for 12 h. After completion of the reaction (monitored by TLC), the solvent was evaporated. The residue was purified by silica gel column chromatography (EtOAc/hexane) to give the desired compound.

Theoretical Calculations
All calculations were carried out using density functional theory (DFT) at the B3LYP (Becke 3-parameter-Lee Yang-Parr) with the basis set of 6-311G (d,p).
Cartesian cooordinates and total energy of optimized structure Figure S62.Optimized geometry by semi-emprical calculation using PM6 of compound 9 Table S1. Atom coordinates and total energy of the optimized geometry of compound 9 Total Energy: -1134.366945 hartrees zero imaginary frequency    Figure S64.Optimized geometry by semi-emprical calculation using PM6 of compound 33 Table S5. Atom coordinates and total energy of the optimized geometry of compound 33